I would err on the larger size (45322 instead of 45321) because a too-big cover is just sloppy while a too-small cover won't go on.
Another approach is to use a more precise measurement, from the tire manufacturer's specs. If the 2012 19' used the currently specified ST205/75R15 tire size, that should be 27.1"... so again I would go with the larger size.
You can certainly pad a too large tire cover. But one that fits just looks better. There's been plenty of posts on the CasitaForum where folks talk about the tire cover developing diaper droop Now, just try to get that visual out of your brain for the rest of the day.
Oh wait! I thought you were talking about the spare tire cover. NEVER MIND...